High WAF-score (Wife Acceptance Factor). It has to be easy to use for both me and my wife. She's no idiot by any means (on the contrary), but she doesn't see any reason to have to fight the box when all she wants to do is watch TV or a DVD. I tend to agree. Tinkering with software and hardware is fun up to a certain point, but when you just want to watch that important game, the last thing you want to have to do is to have to get the machine working right.
Silent. I usually don't care that much if my main computer is noisy, but any machine in the living room has to be as silent as possible.
Powerful. While I don't intend to use my Media PC for gaming, I want it to be powerful enough to show HD-quality video.
Stylish. It has to fit in the living room.